When preparing OE devices like OFETs or OPV cells, in particular flexible devices, usually printing or coating techniques like inkjet printing, roll to roll printing, slot dye coating or flexographic/gravure printing are used to apply the OSC layer. Based on low solubility of the most of the present organic compounds useful as OSC these techniques need the use of solvents in high amounts. In order to reduce solvent de-wetting and to increase dry film levelness surfactants can be used. These additives are especially needed with regard to small molecular OSC or polymeric OSC having a low molecular weight. The use of conventional surfactants or wetting agents is disclosed, e.g. in WO 2009/049744. However, no explicit examples are mentioned. Based on the low solubility of the most of the OSC materials the amounts of surfactants needed are high in relation to the amount of OSC material in the ink formulation.
Furthermore, WO 2009/109273 describes compositions comprising special solvents in order to achieve specific viscosity. The specific viscosity is needed to apply the composition via application methods as roll to roll printing, slot dye coating or flexographic/gravure printing etc. without the need of using high amounts of polymeric binder. However, according to some embodiments of these compositions, these binders can be used as optional components. In addition thereto, the compositions may comprise wetting agents as mentioned above.
Moreover, JP 2001-288416 discloses a coating liquid composition dissolved or dispersed in at least two organic solvents comprising a liquid solvent and a solid solvent. Using a combination of a liquid solvent and a solid solvent provides a thin film having a functional material in a highly dispersed isotropic state.
The OE devices as disclosed in WO 2009/049744, WO 2009/109273 and JP 2001-288416 show useful efficiencies and lifetimes. However, it is a permanent desire to improve the performance of the OSC layer, such as efficiency, lifetime and sensitivity regarding oxidation or water.
In addition thereto, the formation of films having a high leveling is difficult without the use of high amounts of wetting agents. However, these wetting agents may have some drawbacks regarding the performance of the films formed.
Furthermore, the production of multilayer devices based on the technology as mentioned above is difficult to achieve. In order to apply an additional layer on an existing layer, generally the composition subsequently applied comprises a solvent which does not dissolve the existing film. Such approach is usually called orthogonal solvent approach. However, if the organic semiconducting compounds of the existing film and the semiconducting compounds of the subsequently applied composition have similar properties and are soluble in similar solvents such approach is difficult to achieve.
It is therefore desirable to have improved compositions comprising an OSC that are suitable for the preparation of OE devices, especially thin film transistors, diodes. OLED displays and OPV cells, which allow the manufacture of high efficient OE devices having a high performance, a long lifetime and a low sensitivity against water or oxidation. One aim of the present invention is to provide such improved compositions. Another aim is to provide improved methods of preparing an OE device from such compositions. Another aim is to provide improved OE devices obtained from such compositions and methods. In particular, another aim is to provide improved methods for preparing multilayer devices. Further aims are immediately evident to the person skilled in the art from the following description.
Surprisingly it has been found that these aims can be achieved, and the above-mentioned problems can be solved, by providing methods, materials and devices as claimed in the present invention, especially by providing a process for preparing an OE device using a composition being solid at a temperature of 25° C. and fluid at a higher temperature and the boiling point of the solvent is at most 400° C.